Lead-210 and radon-222 anomalies in Mont Blanc snow, French Alps

In the Mont Blanc area, French Alps, the 210Pb deposition flux density estimated from snow cores reaches an unexpected 8400 Bq m−2 yr−1, up to eighty times the mean regional values (110 Bq m−2 yr−1). After an eight-day measurement campaign in February 1998, including the implementation of radon monitors in the Mont Blanc summit snow, we concluded that this anomaly is due to diffusion and temperature-driven convection through the firn of 222Rn emanating from the underlying rocks. During enhanced releases of radon gas, the short-lived radon progeny build-up in the snow is so significant that it could be readily detected from the snow surface merely by gamma prospecting. The thinness and permeability of the snow blanket, the crevasse known to be below the measurement location and the granitic bedrock strongly back up our conclusions, which exemplify potential flaws in experimental methodologies associated with using snow as a passive collector for determining natural radionuclide inputs from the atmosphere